1. Field Of The Invention
This invention relates generally to mixed solvent systems for treating various gas streams containing acid gases. More specifically this invention relates to a solvent composition for treating a gaseous mixture where the major constituents to be removed are CO.sub.2 and H.sub.2 S. The solvent composition maintains single phase integrity under operational conditions and has a tertiary amino alcohol, a physical solvent, and water.
2. Description Of The Prior Art
Gas scrubbing systems are used to removed acidic gases including CO.sub.2, H.sub.2 S, SO.sub.2, SO.sub.3, CS.sub.2, HCN, COS, and sulfur derivatives of C.sub.1 to C.sub.4 hydrocarbons from gas streams. Gas streams, from which these acidic gases must be removed can be from many sources. The gas streams contemplated as having the most significant commercial use for this invention are gas streams from natural gas wells. The gas removed from natural gas wells is rich in methane and other combustible gases, but contains concentrations of acidic gases such as H.sub.2 S and CO.sub.2. Unacceptable concentrations of H.sub.2 S precludes pipe line shipment of the natural gas because of environmental considerations and government regulation. High concentrations of CO.sub.2 in natural gas reduce the heating value of the gas because CO.sub.2 is not combustible.
The processes to scrub the gas as received from the well head fall into two basic categories. The first category involves an apparatus in which the feed gas enters and absorber tower. The feed gases enter the absorber tower at temperatures generally between 50.degree. F. and 100.degree. F. and at pressure of about 400 to 1200 psia where they are contacted with a liquid solvent mixture at temperatures between 20.degree. F. and 180.degree. F. The solvent in the absorber tower removes the acidic gases from the methane gas before the methane exits the tower. The acid gas rich solvent is then fed to a vessel or "flash tank" in which there is a reduced pressure of about 80 to 600 psia. In this second vessel the acid gas rich solvent "flashes" and the acid gases are liberated into a gas phase. Residual methane gas is removed from the vessel and can be used to power the system. The acid gas rich solvent is then fed to a second vessel or flash tank of reduced pressure of a slight vacuum to about 20 psia. In this second flash tank the acid gas concentration in the solvent is reduced or removed completely and the "regenerated" solvent is fed back to the absorber tower. Gas scrubbing systems using this arrangement are generally referred to as "pressure swing processes".
The second category of scrubber systems can be identified as conventional systems for removing acid gases from gas streams. These systems involve an absorber tower which is at a high pressure and a low temperature. The feed gas enters this absorber tower and is contacted with a solvent. The acid gases are removed from the primary gas, for example methane, and the primary gas or methane is removed from the absorber tower. The acid gas rich solvent passes through a heat exchanger where the heat from the hot, regenerated solvent is recovered. It then enters a stripper column, which is at a high temperature and a low pressure. The acid gases are removed or "stripped" from the solvent and the solvent is then cooled and recycled back to the absorber tower. A conventional system can use the same solvents as a pressure swing system. The disadvantage with using a conventional system is that the system requires more energy. The solvent mixture in the stripper column of a conventional system must be heated to its boiling point. In contrast the solvent mixture of a pressure swing system is generally not heated.
Gas scrubbing systems which remove acidic gases from a gas stream, but are not concerned with the treatment of natural gas from a well head, can include synthesis gas operations. In these operations a hydrocarbon feed stock is heated in the presence of water over a catalyst in a vessel commonly referred to as a reformer. The resulting gas stream contains mainly CO, CO.sub.2, and H.sub.2. In the case of ammonia production this gas stream would be mixed with normal air in proportion so as to obtain a ratio of hydrogen and nitrogen suitable for ammonia production. The CO.sub.2 still present in the gas stream can be removed by a scrubbing system containing a solvent as used in the systems for removing acid gas from natural gas well heads.
There are three general categories for solvents used in systems as discussed above. The first category is generally referred to as an aqueous amine solvent where a relatively concentrated amine solution is used for the absorption of acid gases. The second category is generally referred to as an aqueous base scrubbing solvent. In this category the solvent only contains a small amount of an amine as an activator. The third category is generally referred to as a nonaqueous solvent. This solvent contains water as a minor constituent, for example, usually less than 20% of the total concentration of the final solvent mixture. The present invention relates to the third category of gas scrubbing solvents.
There are numerous examples of nonaqueous solvents. In general, these solvents contain an amine compound, a physical solvent, and water. The solvents present in the prior art do not allow for efficient gas scrubbing operations for one or more of the following reasons. Many of the solvents separate into two distinct liquid phases at pressures, temperatures, or gas concentrations that can be found in normal operations of scrubbing systems. Some solvents in the prior art do not allow for efficient operations because they lack the proper selectivity for CO.sub.2 over CH.sub.4. The most significant factor which leads to inefficiency in scrubbing systems occurs when a solvent has too high of a heat of reaction or a poor "working capacity".
In references concerning solvents used in gas scrubbing systems, working capacity can be defined in one of two ways. In a pressure swing system the "effective working capacity" of the solvent is defined as the difference between the concentration of acid gases in solution at equilibrium with the feed gas and the concentration of acid gases in solution at equilibrium with the flash gases generated during an adiabatic flash of the loaded solvent to a reasonable pressure of less than 20 psia. In conventional systems the working capacity can be defined as the difference between the concentration of acid gases in solution in equilibrium with the incoming feed gas and the concentration of acid gases in solution at stripper conditions of high temperatures. A solvent having a high working capacity as measured in a pressure swing system is desirable because of the overall energy efficiency that a pressure swing system has when compared to the conventional system. The solvent mixtures according to this reference encounter difficulties with phase separation under normal operating conditions, have too high of a heat of reaction for efficient gas scrubbing, and/or have too low an effective working capacity.
Solvents that use tertiary amines in combination with physical solvents with and without rate promoting additives have been found to be efficient in acid gas scrubbing processes. In particular, U.S. Pat. No. 4,100,257 describes the use of a solvent mixture for treating acidic gaseous mixtures consisting of a sterically hindered amine, a tertiary amino alcohol, and a physical solvent. This reference states that the sterically hindered amine, due to its unstable carbamate, increases the working capacity of the solvent mixture. The definition of working capacity used in this reference is the difference between the concentration of acid gases in solution in equilibrium with the incoming feed gas and the concentration of acid gases in solution in a stripper column at an elevated temperature. This reference fails to appreciate that it is not enough to increase the solubility of acid gases in a solvent to make a process employing that solvent more energy efficient. Those solvent mixtures do not regenerate on pressure reduction alone. A solvent which is truly more energy efficient must possess two key qualities, which are a high effective working capacity and a low heat of reaction.
The concept of utilizing a solvent with a high working capacity and a low heat of reaction to absorb acid gases in a cyclic process, where the majority of solvent regeneration is achieved by pressure swing alone, is not novel. U.S. Pat. No. 2,649,166 discusses such a process utilizing dialkyl ethers of polyethylene glycol as the solvent. This reference discusses a class of solvents which are known as "physical solvents". For nearly all commercially important feed gas conditions, physical solvents must operate well below ambient temperature because of the relatively poor solubility that these solvents have for acid gases at high temperatures. This requires the use of refrigeration compressors and extensive insulation to minimize heat leaks in the process. Further, for those cases where low acid gas specifications must be met these solvents often must be regenerated through the use of an external heat source. This carries a high penalty in energy efficiency, because a large portion of this heat must later be removed by the refrigeration unit. Another draw back of physical solvents is that a substantial amount of product gas is absorbed along with the acid gases. This requires additional capital and operating expenses to recover the absorber product.
British Pat. No. 1,560,905 discusses the use of an absorption rate promoted methyldiethanolamine (MDEA) solution in a pressure swing process. This reference discusses the process without the use a cosolvent. In this case the heat of solution of this solvent is that of MDEA alone. Further, the position and shape of MDEA's solubility curves requires a gas scrubbing system according to this invention to operate at elevated temperatures which reduces the absorption capacity of the solvent and causes substantial quantities of water vapor to be liberated during flash regeneration. The liberation of water vapor represents a loss of heat from the process to the surroundings which must than be replaced by an external heat input.
British Pat. No. 1,560,905 also discusses the use of an absorbtion rate promoted MDEA solution in combination with physical solvents. It is stated that the combined concentration of the preferred promotor, piperazine, and physical solvent must be kept relatively small if the formation of piperazine carbamate is to be avoided. It is stated that at least 60 percent by weight of water must be present to prevent the precipitation of the carbamate. At this water concentration no significant quantity of "physically" dissolved carbon dioxide can be in solution.
It is desirable to operate a pressure swing process utilizing a solvent which has a high capacity for acid gases at near ambient temperatures, thereby eliminating the need for refrigeration compressors and extensive insulation. Adjustments for solvent temperature changes caused by heat leaks in or out of the system can then be accomplished by conventional means. Further, solvent regeneration through the use of an external heat source would no longer carry a high penalty since this heat would not have to be removed by refrigeration units.
It is an object of this invention to provide a solvent mixture having a tertiary amino alcohol, a physical solvent, and water that has (1) a high working capacity, (2) a single liquid phase at all operating conditions, (3) a comparatively low heat of reaction, (4) a high selectivity of CO.sub.2 over CH.sub.4, and (5) a viscosity that allows the solvent to be used in gas scrubbers.